Can you tell luminance from illuminance? Colour temperature from colour rendering? These are some of the terms you’ll encounter when it comes to installing outdoor lighting. Below, we’ve defined the must-know outdoor lighting terminology, including:
- Colour-rendering index
- Colour temperature
Defining and Measuring Light
Whereas luminance is an objective measurement of the intensity of light, brightness is a subjective perception of intensity. An individual’s perception of brightness depends both on the actual luminance of the light and the surrounding environment. For example, consider the optical illusion that presents surfaces with the same luminance against white and black backgrounds; though identical, these surfaces evoke different brightness impressions based on the surrounding luminance.
Colour-Rendering Index (CRI)
Colour-rendering index (CRI) measures how accurately a light source shows the colours of objects it illuminates compared to natural light. For example, the maximum potential CRI value is 100; the standard value for parking lot lighting is 65. CRI is important in outdoor lighting for spaces like car dealerships, where the colour of the products can have a significant impact on a customer’s decision to buy.
Colour Temperature and Correlated Colour Temperature
Colour temperature describes the colour appearance of a light source. With incandescent lamps and other thermal radiators, colour temperature corresponds with the actual temperature of the light source.
When it comes to gas lamps and solid-state lighting, where the temperature has no relation to the colour of light, colour temperature is measured by comparing the appearance to the light of a thermal radiator. The technical term for this is correlated colour temperature.
Colour temperature is measured in Kelvins (K). For example, the flame of a candle emits yellow light at a temperature of approximately 2,000 K; the filament of an incandescent bulb is yellow-white at 2,700 K; the sun at noon appears at around 5,000 K.
Efficacy describes the energy-efficiency of a light source based on the ratio between its luminous flux and power consumption.
Efficacy is measured in lumens per watt (lm/W). For example, a typical fluorescent lamp is 10 lm/W; a high-flux LED is 120 lm/W.
Luminous flux, or lumens, describes the amount of light a light source radiates each second under standard conditions. It is among the most common (and commonly misunderstood) measurements listed in lighting datasheets and packaging. Lumens do not describe the colour or intensity of light; only how much light the source emits.
Luminous flux is measured in lumens (lm or Φ).
Glare is an effect that occurs when the light is too bright compared to its background, light causing discomfort and reducing the ability to see. Though there are methods of calculating and predicting glare, glare it is also subjective; for example, senior citizens may have more difficulty adjusting to glare than younger viewers.
With consideration in lighting design, glare can be reduced and controlled through proper luminaire placement and use of cut-off fixtures and shields.
Illuminance measures the amount of light that falls on a surface. This measurement has largely replaced terms such as foot-candles, illumination value and illumination level. It is typically described regarding ratios of maximum-to-minimum and average-to-minimum illuminance in an area. Measuring illuminance is necessary to calculate other measurements like luminance and glare.
Illuminance is measured in luxes (lx), with one lux equalling one lumen of incident light per square metre of light-receiving surface. For example, a typical bright summer day is approximately 100,000 lux; an overcast sky would clock in at 5,000 lux; a living room may have 100 lux. One foot-candle is equal to 10.76 lux.
Luminance measures the intensity of light emitted by an object or surface. This measurement can apply to light that emits from a lamp or luminaire or a surface that reflects or transmits light, such as a window or road surface. Usually, luminance is measured from the perspective of an observer looking towards the lighted area.
Luminance is measured in candelas per square metre (cd/m2). For example, an office desk might reflect 100 cd/m2 of a desk lamp’s light; a typical fluorescent lamp may emit luminance of 5000 to 15,000 cd/m2; the surface of the sun has a luminance of 1650 cd/m2.
Luminous intensity measures the amount of light (the lumens or luminous flux) per second emitted in a specific direction.
Luminous intensity is measured in candelas (cd).
Watts (W) measure the amount of energy a lamp consumes to produce light. The lower the wattage, the less energy the lamp requires. With incandescent lamps, and a lamp with higher wattage produces more light; however, that is not the case with compact fluorescent lamps nor LEDs.
Types of Outdoor Lighting and Outdoor Lighting Components
A ballast is a device that controls the electrical current used to produce light in a gas discharge lamp such as a fluorescent light. If the ballast fails, the lamp can overheat and burn out.
A bollard is a type of lighting fixture that includes a short, ground-mounted post topped with a light source that is directed downward.
A driver controls the electrical current in a solid-state lamp, ensuring a consistent voltage level as the number of LEDs in the circuit increases or decreases. The driver will increase or decrease the voltage as necessary to maintain a constant current.
A type of lamp that produces light inside a transparent, gas-filled tube. Each end of the lamp is sealed with an electrode: one positively-charged (the anode) and the other negatively-charged (the cathode). When the light is switched on, voltage travels between these two electrodes, and fast-moving electrons collide with gas molecules to produce light.
Fluorescent lamps are approximately eight times as efficient as incandescent lamps and last far longer.
Halogen Incandescent Lamp
A type of lamp that produces light by heating a filament inside of a gas-filled bulb. The difference between a halogen incandescent and normal incandescent lamp is a reaction between the halogen gas and evaporated filament material – part of the evaporated material returns to the filament, prolonging the life of the lamp. As a result, incandescent halogen lamps have a considerably longer lifespan than normal incandescent lamps, lasting between 1,000 and 6,000 hours of use.
A type of lamp that produces light by heating a filament inside of a bulb. Only 5% of the energy an incandescent bulb uses goes towards producing light, while the rest produces heat. Because of this, incandescent lamps are largely recognized as the least energy-efficient forms of outdoor lighting.
Since the filament evaporates over time, incandescent bulbs have a relatively short lifespan compared to other types of outdoor lighting, lasting around 1,000 hours of use at most.
Light-Emitting Diode (LED)
A light-emitting diode (LED) is a tiny microchip containing semi-conductive material that produces light by applying a voltage to an electrical junction. Different types of semi-conductive materials produce different colours of light, making it possible to produce a wide range of different colour LEDs. If the chip or cluster of chips is encapsulated in a bulb, it is an LED lamp.
The technical term for a light fixture or light fitting; in other words, the apparatus containing a light source.
A reflector reflects, refracts, absorbs or transmit lights to efficiently direct the light emitted from the bare lamp. It may be a part of the luminaire or the lamp itself, or a component added separately during or after installation. Reflectors are important in the types of outdoor lighting that require precise light control, such as floodlights, spotlights and road lights.
Solid-state lighting is a general term for electronic light sources using solid, semi-conductive material. Light-emitting diodes or LEDs are a type of solid-state lighting.
Lighting Design Principles
Mounting Height (MH)
The vertical distance between the base of a pole and the luminaire; or the ground and the luminaire, in the case of a wall-mounted light.
Light pollution, also known as an uplight, refers to light unnecessarily directed upward into the night sky. Excessive light pollution creates an unnatural glow that masks the view of the sky from the ground. Effective outdoor lighting design uses shields, hoods and other devices can reduce or eliminate skyward lighting.
Light trespass, also known as a backlight, refers to light spilling into unwanted areas like adjacent homes or properties. It can be avoided through careful lighting design, including proper location, mounting and shielding of luminaires. Many cities have by-laws limiting the level of light allowed near property lines to reduce light trespass.
Ask an Outdoor Lighting Installation Expert
When it’s done right, outdoor lighting can boost visibility, increase safety and enhance the night-time environment overall. Poor lighting design, on the other hand, can result in excessive glare, wasted energy and complaints about light pollution and light trespass. Ask an expert on outdoor lighting installation before you embark on your next big lighting project.